1. Field of the Invention
This invention relates to a fuel supply control apparatus for an internal combustion engine capable of detecting an air intake quantity of an internal combustion engine by means of an air flow sensor to control a quantity of fuel to be supplied to the internal combustion engine on the basis of the detected output.
2. Description of the Prior Art
FIG. 1 is a schematic diagram showing a suction or intake air system of an internal combustion engine. Referring now to FIG. 1, a suction air, which has passed through an air cleaner 10, is sucked into an internal combustion engine 1 through an air flow sensor 13 and a throttle valve 12. And, in order to control a fuel quantity in the internal combustion engine 1, an air intake quantity for one intake stroke is determined from an output of the air flow sensor 13 arranged upstream of the throttle valve 12 and the rotation frequency of the engine and a quantity of a fuel to be supplied to the engine is controlled on the basis of the determined air intake quantity per each suction stroke.
However, since blow back or back flow air occurs from the internal combustion engine 1 in the case where the throttle valve 12 is almost completely opened, a quantity of the blow back of back flow air is detected by the air flow sensor 13, whereby the output of the air flow sensor 13 is larger than the quantity of air which is really sucked in the internal combustion engine 1. As a result, in the case where the quantity of a fuel to be supplied is controlled on the basis of an output from the air flow sensor 13, a problem has occurred in that air-fuel mixture is over-rich.
FIG. 2 is a graph showing a relation between a suction pressure (axis of abscissa) and an air intake quantity (axis of ordinate). Referring to FIG. 2, reference mark a designates an output from the air flow sensor 13 and reference mark b designates a quantity of air which is really sucked into the internal combustion engine 1. In addition, FIG. 3 is a graph showing a relation between a time t (axis of abscissa) and a volume of suction air V (axis of ordinate) in the case where the throttle valve 12 is almost completely opened. Referring to FIG. 3, reference mark a designates a quantity of suction air while reference mark b designates a quantity of blow back or back flow air. As understood from FIGS. 2, 3, in the case where the throttle valve 12 is almost completely opened, the blow back or back flow of air from the internal combustion engine 1 is detected by the air flow sensor 13, whereby the air intake quantity for the internal combustion engine 1 can not be accurately detected, and as a result, the above described problem occurs.
FIG. 4 is a graph showing a relation between a rotation frequency Ne (axis of abscissa) of the internal combustion engine 1 and an air intake quantity Qc (axis of ordinate) in the case where the throttle valve 12 is completely opened. The air intake quantity is varied with the number of revolution Ne of the internal combustion engine 1. In addition, since the suction air is warmed in the suction system, the density of the suction air is varied with temperature in the suction system. Furthermore, in the case where water temperature in the internal combustion engine 1 is low, the suction air is warmed to a less extent, whereby the packing efficiency of the suction air is raised. Also in the case where the temperature of the suction air is high, a temperature-rise of the suction air is small, so that the packing efficiency is raised.
Accordingly, in the case where such parameters of the internal combustion engine 1 are disregarded and an output from the air flow sensor 13 is used in the foregoing manner to control fuel, a problem occurs in that the air-fuel mixture is over-enriched.